In a conventional remote control receiving system, infrared control signals are transmitted between a remote control source and receiver. The signals are characterized by custom codes and carrier signals. As illustrated in the block diagram of FIG. 1, the infrared control signals are received together with external disturbances, for example optical noise and electrical noise, and the received signals are converted to electrical signals by a photo-detector 11. The converted electrical signals are processed and amplified by a head amplifier 12. The amplified signals, including infrared control signals, optical noise and electrical noise are limited to a predetermined magnitude by a limiter 13. A band pass filter (BPF)/band rejection filter (BRF) 14 attenuates the level of the external noise and amplifies the control signal. An envelope detector 15 tracks the portion, or envelope, of the control signal which has passed through the BPF/BRF 14, using the difference in level between a control signal interval and a NULL interval (noise interval). A comparator 16 reproduces the custom code from the detected envelope.
When external noise is received at photodetector 11, the head amplifier 12 amplifies not only the control signal, but also the noise as well, which can lead to system malfunction. In some cases, the head amplifiers 12 may include a bandpass filter to eliminate some of the noise. However, where the external noise is of a frequency similar to that of the infrared control signal carrier of 38 KHz, for example noise generated by a second harmonic of a TV horizontal fly-back signal or an inverter fluorescent lamp signal, that external noise can still interfere with system operation.
When the frequency of the noise is within the passband of the filter, or when the level of the noise is relatively higher than the infrared control signal, the signal passing through the limiter 13 may include more than the signal carrier, making it difficult to distinguish an envelope from the limiter output. Accordingly, a second harmonic of a TV horizontal fly-back signal or an inverter fluorescent lamp signal may still cause system malfunction. In some systems, the envelope of the control signal and noise is reproduced using an automatic gain controller instead of the limiter. However, in the case where the noise has a frequency similar to that of the control signal, the system exhibits the same shortcomings as a system employing a limiter. Furthermore, an automatic gain controller involves a complicated circuit, which, in an integrated configuration, increases the surface area occupied by the chip.